1. Field of the Invention
The present invention relates to electrically conducting organic polymers and, more particularly, a one-step chemical method of making charge-transfer acceptor doped poly N-alkylcarbazoles together with the resulting new conducting polymeric organic semiconductors.
2. Description of the Prior Art
High molecular weight organic polymer materials are generally non-conductive because they do not have free electrons like metals. It has been found, however, that certain high molecular weight materials having intrinsic double bond structures such as polyacetylene, polythiazine and polypyrrole may become highly conductive when doped with certain electron acceptors or donors. These compounds have proved to be of a great deal of interest inasmuch as they may combine some of the traditional properties of organic polymers such as high strength, light weight, flexibility and low temperature processing together with selective electrical properties including high electrical conductivity. In addition, their cost is relatively low.
Such materials undoubtedly will have an important impact on many areas of technology, especially the electronics industry. For example, experimental batteries made from conducting polymers have been shown to exceed current power sources in both power and energy densities. Other areas of potential applications include chemical or gas sensors, low cost, large area optical sensors, switches, light weight electrical connections, wire, and in their film form for many types of microelectronic circuits and large area solar cells.
Thus, organic materials that behave as metals or semiconductors will provide the advantages of these materials together with additional advantages of being soluble in organic solvents or having low melting points and glass transition temperatures which both minimize the cost of processing and permit composites to be made with thermally sensitive materials such as doped Si or GaAs, for example. The enormous molecular design flexibility of organic chemistry enables precise tailoring of properties to fill a wide range of applications as enumerated above. In addition, the high strength and conductivity-to-weight ratios lend the advantage of fabrication of many electrical devices of much lower weight than conventional materials.
In the prior art, a large number of polymeric conductors have been made. These include polyacetylene and its analogues which may be doped with I.sub.2, AsF.sub.5 and BF.sub.4.sup.- or the like. In addition, various phenylene polymers and phthalocyanine complexes have been synthesized as conductive materials.
Highly conducting p-type materials have been obtained by doping the polymer with a charged transfer acceptor such as I.sub.2 or AsF.sub.5 from the gas or with ClO.sub.4.sup.- or BF.sub.4.sup.- by electrochemical oxidation. An n-type material has been achieved by a doping with alkali metal. In known cases of these two types of materials, however, to date only the p-type show any environmental stability.
Theoretically, conductivity takes place both along the polymer chain and between adjacent chains. The active charge carrier, at least in the aromatic materials, is believed to be a bipolaron that is delocalized over several monomer units. The mobility of such a species along the polymer chain is reduced by conformational disorder, necessitating a rigid highly crystalline chain structure for maximum intrachain conductivity. Various mechanisms such as "hopping" and "interchain exchange" are thought to be responsible for the interchain part of the conductivity. Unfortunately all of the most highly crystalline polymers of high conductivity are insoluble and infusable. Such is the case with the most common prior art conducting polymer, polyacetylene, which because of this, must be used in the same form as polymerized. In film form it becomes highly porous fibrillar networks which are tough, cheap, and can be electrochemically doped very rapidly. Polyacetylene films have been used in light weight storage batteries and can also be used to make Schottky barriers which exhibit a photovoltaic effect.
Successful environmentally stable doped conducting polymers are described in two co-pending applications, the first, U.S. Pat. No. 4,452,725 to S. T. Wellinghoff, S. A. Jenekhe (an inventor in the present application) and T. T. Kedrowski concerns conducting polymers of N-alkyl 3,3'carbazolyl chemically doped with charge transfer acceptor dopants such as the halogens. The second, Ser. No. 525,763 to S. A. Jenekhe (an inventor in the present invention) S. T. Wellinghoff and Y. A. Chen filed of even date herewith concerns complexes of poly (N-alkyl phenothiazine) doped with charge transfer acceptors.
In the prior art electrochemical synthesis of electrically conducting polymers the requisite monomers and dopants are dissolved in a solvent and the resulting solutions are electrolyzed by application of electrical power. Doped electrically conducting polypyrrole, polythiophene, polyazulene, polypyrene, and others have been successfully prepared by electrolysis of solutions of their monomers with such dopant species as ClO.sub.4.sup.-, BF.sub.4.sup.-, PF.sub.6.sup.-, etc. Simultaneous polymerization and doping take place, thus producing the doped conducting polymer in one step. However, compared to chemical synthesis far fewer conducting polymers are available by electrochemical synthesis.
In the prior art chemical synthesis of electrically conducting polymers two distinct steps are usually required. First, the base polymer in the form of powder, pellet, or film is produced by polymerization of the appropriate monomer. Where the polymer film is desired and is not produced directly during polymerization, the powder or pellet or whatever prior form may also require processing into a film as an intermediate step. Second, the base polymer film, powder, or other form is chemically or electrochemically doped by exposing and contacting the virgin polymer with suitable dopant in the vapor or liquid phase. This prior art two-step method of making electrically conducting polymers is exemplified in the preparation of doped p-type or n-type polyacetylene films as described by A. J. Heeger et al in U.S. Pat. Nos. 4,204,216 and 4,222,903 (1980). Likewise the prior preparation of many other doped conducting polymer complexes such as those based on poly p-phenylene, poly phenylene sulfide, metal-phthalocyanines, polyquinolines, etc., follows the two-step chemical synthesis procedure.
In the prior art two-step method of synthesizing doped conducting polymers uniformity of doping in the base solid polymer has been difficult to achieve. Nonuniformity of doping and inherently low rates of doping are so partly because of their dependence on the diffusivity of the doping species, the physical form, density, surface area, molecular structure, and crystallinity of the starting base polymer. The chemical doping step may also produce undesired chemical transformations in the starting backbone polymer structure, such as crosslinking, to the extent of precluding further processibility of the doped conducting polymer.
One attempt by L. W. Shacklette et al (J.Chem.Phys. 73, 4098 (1980)) to achieve a one-step method of chemical synthesis of doped conducting polymers consisted in the solid-state polymerization and doping of para-phenylene oligomers which have a degree of polymerization from 2 to 6, i.e., biphenyl, para-terphenyl, para-quarterphenyl, etc., by arsenic pentafluoride AsF.sub.5 vapor. However, the monomer, para-phenylene, does not polymerize with AsF.sub.5. Thus, this is still more or less a two-step method in which first the oligomers are produced from the monomer and secondly the oligomers are further polymerized and simultaneously doped with AsF.sub.5 to yield a doped conducting poly p-phenylene.